Among the functions of conventional electrooptical devices is light deflection. While optical deflectors are used in various devices, most of them operate on mechanical motion. In a laser printer, laser light is deflected by reflection using a polygon mirror which is rotated to continuously change the direction of its mirror face.
In the tracking mechanism of a magnetooptical disk device, a light beam is deflected by moving a lens to the fight or left, or by changing the direction of a reflecting mirror. However, in the above examples, the mechanism is complex, adjustments are difficult to perform in assembly, and it is vulnerable to vibration. The deflection speed is limited by the size and weight of mechanical parts. Furthermore, the power consumption increases with the deflection speed.
To solve the above problems, optical deflectors having no mechanical motion which include a variable diffraction grating using a surface acoustic wave (SAW) device are proposed. In this grating deflector, to deflect a light beam, a SAW device is formed in a waveguide and the lattice interval is varied by changing the acoustic wave frequency. However, this deflector is low in diffraction efficiency and is therefore low in light utilization factor. In addition, this deflecting device is difficult to manufacture. To solve these problems, several liquid-crystal-based deflectors have been proposed which can be manufactured easily and can provide a wide variable range in the refractive index.
For example, a deflector which deflects a light beam by changing the orientation of a liquid crystal by applying a high voltage between two electrodes was proposed in each of A. F. Fray and D. Jones, "Large-Angle Beam Deflector Using Liquid Crystal," Electo. Lett., Vol. 11, pp. 358 (1975) and A. Sasaski and T. Ishibashi, "Liquid-Crystal Light Deflector," Electo. Lett., Vol. 15, pp. 239 (1979). A variable diffraction grating utilizing a DC-current-induced Williams domain was proposed in K. Okano and S. Kobayashi, "Applications of Liquid Crystals" (in Japanese), Baifukan Co., Ltd., pp. 213 (1989). A deflector utilizing the switching by total reflection was proposed in each of G. Labrunie and S. Valette, "Nematic Liquid Crystal Digital Light Deflector," Appl. Opt. Vol. 13, pp. 1802 (1974) and R. A. Kashnow and C. R. Stein, "Total-Reflection Liquid-Crystal, Electro-Optic Device," Appl. Opt., Vol. 12, pp. 2309 (1973). Furthermore, there was proposed a deflector in which an optical path difference is produced by a linearly varying refractive index distribution that is generated by an electric field gradient.
In the deflectors using a high voltage, it is difficult to control the electric field distribution and, therefore, a beam shape is likely to be distorted. In the device utilizing the Williams domain, the light utilization factor is low as in the case of using the SAW device and the deflection angle depends on the light wavelength. The devices utilizing the total reflection can deflect a light beam in only two directions. In the device utilizing an electric field gradient, since a light beam is introduced in parallel with an electric field applied to a liquid crystal, the device needs to be made thinner to improve the response speed, which necessarily reduces the deflection angle. Conversely, to increase the deflection angle, the device becomes thicker, which results in a lower response speed. That is, a dilemma exists between the response speed and the deflection angle.
Another important function of electrooptical devices is focusing. A mechanism for focusing light is employed in many kinds of devices, and in most of such mechanisms a lens is moved mechanically. In autofocus cameras, various types of actuators have been developed to change distances between lenses to thereby change the focal length of a combination of the lenses. In a focusing mechanism for magnetooptical disk devices, the focus point is changed by moving a lens vertically. However, in these focusing mechanisms, the mechanism itself is complex and vulnerable to vibration, and adjustments in assembly are difficult. The focus varying speed has a limitation depending on sizes and weights of mechanical parts. Furthermore, the power consumption increases along with the focus varying speed.
To solve the above problems, liquid-crystal-based variable focus lenses have been proposed which do not involve mechanical motion. With the advantages that the device can be produced easily and that the refractive index can be changed over a wide range, liquid crystals have been applied to a variety of devices. For example, a device in which a liquid crystal is given a lens-like refractive index distribution by forming fine electrodes on a pair of glass plates and properly controlling voltages applied to the respective electrodes was proposed in Patrick F. Brinkley, Stephaen T. Kowel and Chinghua Chu, "Liquid Crystal Adaptive Lens: Beam Translation and Field Meshing," Applied Optics, Vol. 27, pp. 4578 (1989). A variable refractive index Fresnel lens constituted by injecting a liquid crystal between two glass plates, one of which is formed into a Fresnel lens, was proposed in S. Sato, A. Sugiyama and R. Sato, "Variable-Focus Liquid-Crystal Fresnel Lens," Jpn. J. Appl. Phys., Vol. 24, L626 (1985). A device in which a liquid crystal is placed on a condensing diffraction grating formed on a light waveguide was proposed in Technical Materials of the '90 Optical Disk Conference (in Japanese), Optoelectronic Industry and Technology Development Association, pp. 71 (1992). A device for producing a lens effect by forming a non-uniform electric field by means of a concentric electrode pattern was proposed in S. Masuda, T. Nose and S. Sato, "Optical Characteristics and Molecule Orientations in a Hybrid Orientation Liquid Crystal Electrooptical Microlens" (in Japanese), Optics, Vol. 20, pp. 232 (1991).
In the method of controlling the refractive index by fine electrodes, very fine electrodes are required to produce fair wavefronts of output light. In the device using a Fresnel lens and a diffraction grating, a wavelength dispersion occurs because the focal length varies with the wavelength. Furthermore, since a light beam is introduced in parallel with an electric field applied to the liquid crystal, the device needs to be made thinner to increase the response speed, resulting in an increase of the lens focal length. Conversely, the device needs to be made thicker to decrease the focal length, resulting in a slow response speed. That is, a dilemma exists between the response speed and the focal length. In the device of generating an electric field by the concentric pattern, the electric field distribution is difficult to control and a light beam suffers from large aberrations.
To avoid the above-mentioned problem of optical aberrations, Published Unexamined Patent Application No. 5-206241 discloses a device which performs deflection of a light beam by applying a continuously varying electric field to a nematic liquid crystal utilizing its property that the refractive index changes linearly with respect the electric field.
Nematic liquid crystals have a problem in that the change of the orientation of the liquid crystal molecular axes takes a long time from the application of an electric field, because they do not have spontaneous polarization, a property that a material itself is polarized without application of an external electric field. That is, this type of device is inferior in the response speed and, therefore, not suitable for applications which require a high response speed, such as a switching element used in optical communication.
In ferroelectric liquid crystals, polarization vectors of liquid crystal molecules incline so as to match the direction of an electric field in the smectic-A phase (electric-field-induced inclination). In the case of nematic liquid crystals, however, polarization occurs only after the application of an electric field. In terms of response speed, liquid crystals having spontaneous polarization are superior to nematic liquid crystals. In addition, since the response speed of the orientation with respect to the electric field increases with the magnitude of polarization, the use of a liquid crystal having a large spontaneous polarization is effective in improving the response speed.
Furthermore, it should be considered that the response speed decreases with the operation voltage. In conventional deflectors using a nematic liquid crystal, in which an electric field is applied parallel with the light incidence direction, the optical path length within the liquid crystal needs to be increased to obtain a large deflection angle. Therefore, the electrode interval becomes long, and the operation voltage needs to be increased to generate a desired electric field.
Therefore, an object of the present invention is to improve the response speed of a deflector using a nematic liquid crystal while maintaining its advantages that no disorder occurs in the wavefronts of a deflected light beam, the light utilization factor is high, and there is no mechanical parts, i.e., moving parts.
More specifically, an object of the invention is to provide a deflector which can be driven by a small operating voltage and has a high response speed, by removing the correlation between the deflection angle and the operation voltage by using a ferroelectric liquid crystal having spontaneous polarization, and employing an arrangement in which a light path direction and an electric field direction are not in parallel with each other.